Obstacle traversing wheelchair

ABSTRACT

The present invention provides a curb-climbing wheelchair having pivotal arm assemblies. The pivotal arm assemblies include at least one front caster and have the ability to rotate or pivot. The pivot arms rotate or pivot in response to moments generated by accelerating or decelerating the wheelchair thereby raising or lowering the front caster(s) through the rotational movement of the pivot arms. By raising or lowering the front caster(s) in such a manner, curb-like obstacles can be traversed in a low-impact manner.

CROSS REFERENCES TO RELATED APPLICATIONS

[0001] This patent application is a continuation of U.S. patentapplication Ser. No. 09/698,481, filed on Oct. 27, 2000, and titled“Obstacle Traversing Wheelchair.”

FIELD OF THE INVENTION

[0002] The invention relates generally to wheelchairs, and moreparticularly, to a wheelchair having pivotal assemblies for traversingobstacles such as curbs and the like.

BACKGROUND OF THE INVENTION

[0003] Wheelchairs are an important means of transportation for asignificant portion of society. Whether manual or powered, wheelchairsprovide an important degree of independence for those they assist.However, this degree of independence can be limited if the wheelchair isrequired to traverse obstacles such as, for example, curbs that arecommonly present at sidewalks, driveways, and other paved surfaceinterfaces.

[0004] In this regard, most wheelchairs have front and rear casters tostabilize the chair from tipping forward or backward and to ensure thatthe drive wheels are always in contact with the ground. One suchwheelchair is disclosed in U.S. Pat. No. 5,435,404 to Garin. On suchwheelchairs, the caster wheels are typically much smaller than thedriving wheels and located both forward and rear of the drive wheels.Though this configuration provided the wheelchair with greaterstability, it made it difficult for such wheelchairs to climb overobstacles such as, for example, curbs or the like, because the frontcasters could not be driven over the obstacle due to their small sizeand constant contact with the ground.

[0005] U.S. Pat. No. 5,964,473 to Degonda et al. describes a wheelchairhaving front and rear casters similar to Garin and a pair of additionalforward lift wheels. The lift wheels are positioned off the ground andslightly forward of the front caster. Configured as such, the liftwheels first engage a curb and cause the wheelchair to tip backwards. Asthe wheelchair tips backwards, the front caster raises off the ground toa height so that it either clears the curb or can be driven over thecurb.

[0006] While Degonda et al. addressed the need of managing a frontcaster while traversing an obstacle such as a curb, Degonda et al. isdisadvantageous in that additional wheels (i.e., lift wheels) must beadded to the wheelchair. Hence, it is desirable to provide a wheelchairthat does not require additional lift wheels or other similar typemechanisms to raise a front caster off the ground to a height so thatthe caster either clears an obstacle or can be driven over the obstacle.

SUMMARY OF THE INVENTION

[0007] According to a general embodiment of the present invention, awheelchair for traversing obstacles is provided. The wheelchairincludes, for example, a frame, a pivoting assembly, and a driveassembly. The pivoting assembly has at least one pivot arm having afirst portion, second portion and third portion. The first portion ispivotally coupled to the frame. The second portion has at least onecaster attached thereto. The drive assembly is coupled to the thirdportion of the pivot arm. In operation, the pivot arm pivots in responseto the forces generated by the drive assembly, which is coupled to thepivot arm. As used herein, when two objects are described as beingcoupled or attached, it is applicants' intention to include both directcoupling and/or attachment between the described components and indirectcoupling and/or attachment between the described components such asthrough one or more intermediary components.

[0008] According to a more specific embodiment of the present invention,a wheelchair for traversing obstacles having for example, a frame and aseat for seating a passenger are provided. Pivotally coupled to theframe are a pair of pivot arms. Each pivot arm has a first distalportion, a second distal portion, and a pivotal connection between thefirst and second distal portions for pivotally coupling the pivot arm tothe frame. A motor is coupled to the first distal portion and a frontcaster is coupled to the second distal portion of each pivot arm. Adrive wheel is coupled to each motor for translating the motor'srotational energy to the ground. At least one rear caster is coupled tothe frame to provide for rear stability. By accelerating the wheelchairforward, the drive wheels generate a moment causing each pivot arm topivot or rotate thereby raising the front casters to a height sufficientto traverse the obstacle.

[0009] According to another aspect of the present invention, a secondembodiment of a obstacle traversing wheelchair is provided. The secondembodiment includes, for example, a frame and a seat for seating apassenger. Pivotally coupled to the frame are a pair of pivot armshaving casters connected thereto. Each pivot arm has a first distalportion and a second distal portion that acts as a pivotal connectioncoupling the arm to the frame. A motor is pivotally coupled to eachpivot arm at a location between the first and second distal portions.The pivotal coupling between the motor and the pivot arm is furtherinfluenced by a resilient member providing suspension between the motorand pivot arm. The motor is preferably a gearless, brushless,direct-drive motor although brush-type motors with transmissions canalso be used. A front resilient assembly is coupled to the frame and themotor's pivotal connection to the pivot arm so as to provide a constantresilient force between the frame, the motor's pivotal connection, andthe arm.

[0010] According to another aspect of the present invention, a method oftraversing one or more obstacles is provided. The method includes, forexample, accelerating a wheelchair toward the one or more obstacles and,through such accelerating, causing a raising of one or more frontcasters by pivoting an arm that is coupled to the one or more frontcasters so that the one or more front casters are raised to a heightsufficient for the one or more front casters to traverse the obstacle.The step of pivoting the arm coupled to the one or more front castersincludes, for example, the step of generating a moment associated withthe pivot arm causing the pivot arm to rotate in the direction ofraising the one or more front casters. The height by which the frontcasters must be raised to traverse an obstacle varies from raising thefront casters to a height where their axles are just above the height ofthe obstacle to raising the front casters to a height where the casters'lower extremities are above the height of the obstacle. In the casewhere the front casters are raised to a height where their axles arejust above the height of the obstacle, the wheelchair engages the frontcasters with the obstacle and drives the front casters there over.

[0011] According to another aspect of the present invention, a method ofdescending curb-like obstacles is also provided. In particular, thepresent invention lowers the front casters over a curb onto the newlower elevation when descending to provide forward stability for awheelchair while the drive wheels and rear caster(s) are still on highercurb elevation. As the drive wheels continue over the curb and contactthe new lower elevation forward stability is still maintained by virtueof the front casters while the rear caster is still on the higher curbelevation.

[0012] It is, therefore, an advantage of the present invention toprovide a cost-efficient wheelchair that can traverse one or morecurb-like obstacles.

[0013] It is, therefore, another advantage of the present invention toprovide a mid-wheel drive wheelchair with pivotable front casterassemblies.

[0014] It is, therefore, a further advantage of the present invention toprovide a torque-based method of raising the front casters of awheelchair for traversing curb-like obstacles.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In the accompanying drawings which are incorporated in andconstitute a part of the specification, embodiments of the invention areillustrated, which, together with a general description of the inventiongiven above, and the detailed description given below, serve to examplethe principles of this invention.

[0016]FIGS. 1 and 2A are front and rear perspective views, respectively,of a first embodiment of a wheelchair of the present invention.

[0017]FIG. 2B is a front perspective view of an alternative embodimentof the wheelchair of FIGS. 1 and 2A having a stabilizing torsionelement.

[0018]FIG. 3 is an exploded perspective view of certain components ofthe first embodiment.

[0019]FIGS. 4A, 4B, and 4C are illustrations showing the forces actingon the wheelchair of the first embodiment in the static, acceleratingand decelerating mode of operation.

[0020]FIGS. 5A, 5B, 5C, 5D, and 5E sequentially illustrate thecurb-climbing operation of the first embodiment.

[0021]FIGS. 6A, 6B, 6C, and 6D sequentially illustrate the curbdescending operation of the first embodiment.

[0022]FIGS. 7 and 8 are front and rear perspective views, respectively,of a second embodiment of a wheelchair of the present invention.

[0023]FIG. 9A is an exploded perspective view of certain components ofthe second embodiment.

[0024]FIG. 9B is an enlarged view of a portion of FIG. 9A showing anassembled drive wheel and caster arrangement.

[0025]FIGS. 10A, 10B, and 10C are illustrations showing the forcesacting on the wheelchair of the second embodiment in the static,accelerating and decelerating mode of operation.

[0026]FIGS. 11A, 11B, 11C, 11D, and 11E sequentially illustrate thecurb-climbing operation of the second embodiment.

[0027]FIGS. 12A, 12B, 12C, 12D, and 12E correspond to enlarge portionsof FIGS. 11A, 11B, 11C, 11D, and 11E, respectively, particularly showingthe sequential range of motion of a front resilient assembly of thepresent invention.

[0028]FIGS. 13A, 13B, 13C, and 13D sequentially illustrate thecurb-descending operation of the second embodiment.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS

[0029] Referring now to the drawings, and more particularly to FIGS. 1and 2A, perspective views of a wheelchair 100 of the present inventionare shown. The wheelchair 100 has a pair of drive wheels 102 and 104,front casters 106 and 108, rear caster 110, and front riggings 112 and114. The front riggings 112 and 114 include footrests 116 and 118 forsupporting the feet of a passenger. The front riggings 112 and 114 arepreferably mounted so as to be able to swing away from the shown centerposition to the sides of wheelchair 100. Additionally, footrests 116 and118 can swing from the shown horizontal-down position to a vertical-upposition thereby providing relatively unobstructed access to the frontof wheelchair 100.

[0030] The wheelchair 100 further includes a chair 120 having a seatportion 122 and a back portion 124 for comfortably seating a passenger.Chair 120 is adjustably mounted to frame 142 so as to be able to moveforward and backward on frame 142, thereby adjusting the passenger'sweight distribution and center of gravity relative to the wheelchair. Inthe most preferred embodiment, chair 120 should be positioned such thata substantial portion of the wheelchair's weight when loaded with apassenger is generally above and evenly distributed between drive wheels102 and 104. For example, the preferred weight distribution ofwheelchair 100 when loaded with a passenger should be between 80% to 95%(or higher) on drive wheels 102 and 104. The remainder of the weightbeing distributed between the front and rear casters. Armrests 126 and128 are also provided for resting the arms of a passenger or assisting apassenger in seating and unseating from chair 120.

[0031] The wheelchair 100 is preferably powered by one or more batteries130, which reside beneath the chair 120 and in-between drive wheels 102and 104. A pair of drive motors 136 and 138 and gearboxes are used topower drive wheels 102 and 104. The motors and their associatedtransmissions or gearboxes (if any) forming a drive assembly. A controlsystem and controller (not shown) interface batteries 130 to the drivemotors 136 and 138 so as to allow a passenger to control the operationof the wheelchair 100. Such operation includes directing thewheelchair's acceleration, deceleration, velocity, braking, direction oftravel, etc.

[0032] Front casters 106 and 108 are attached to pivot arms 132 and 134,respectively. Rear caster 110 is attached to rear caster arm 140. Whileonly one rear caster is shown, it should be understood that in thealternative two rear casters can also be provided. As will be describedin more detail, pivot arms 132 and 134 are pivotally coupled to frame142 for curb climbing and descending, while rear caster arm 140 isrigidly coupled to frame 142.

[0033] Springs 144 and 146 are coupled to the arms 132 and 134 and theframe 142. More specifically, the coupling to arms 132 and 134 ispreferably via attachment to the housings of motors 136 and 138,respectively. The coupling to the frame 142 is via attachment to seatback 124. So configured, each spring provides a spring force urging themotor housings upward and the seat 120 or the rearward portion of frame142 downward.

[0034]FIG. 2B is a partial front perspective view of wheelchair 100showing a torsion bar 200 of the present invention. Beyond a certainrange of motion, torsion bar 200 ensures that arms 132 and 134 influenceeach other. In this regard, torsion bar 200 has a torsion section 206and stem sections 208 and 218. Torsion bar 200 is preferably made bytaking a stock of spring steel and performing two bends in the stock toform torsion section 206 and stem sections 208 and 210. As shown in FIG.2B, arms 132 and 134 have attached thereto first and second torsionmounting elements 202 and 204. Each torsion mounting element includes asemi-circular groove therein for accepting a stem section of the torsionbar 200. The torsion bar 200 is held in place within torsion mountingelements 202 and 204 via forced fit within the semi-circular grooves. Inoperation, arm 132 or 134 is free to independently move (i.e., raise orlower) a limited distance before it influences the other arm via torsionbar 200. More specifically, once the torsion limit of torsion bar 200 isexceeded, it behaves as a substantially rigid member translating anyfurther motion of one arm to the other arm.

[0035] The suspension and drive components of wheelchair 100 are furtherillustrated in the exploded prospective view of FIG. 3. Morespecifically, pivot arm 132 has a base member 306 and an angled member302 extending therefrom. The distal end of angled member 302 includes afront swivel assembly 304 that interfaces with a front caster 106. Basemember 306 has attached thereto a mounting plate 308 for mounting drivemotor 136 and gearbox assembly 309. Drive motor 136 is coupled to pivotarm 132 through gearbox assembly 309 and mounting plate 308. The gearboxassembly 309 interfaces drive motor 136 to drive wheel 102, which ismounted on drive axle 311. The gearbox assembly 309 is preferablyattached to mounting plate 308 with screws or bolts and mounting plate308 is preferably welded to base member 306.

[0036] Pivot arm 132 has a pivot mounting structure between base member306 and angled member 302. The pivot mounting structure includesbrackets 310 and 312 and sleeve 314. Brackets 310 and 312 are preferablywelded to base member 306 and sleeve 314 is preferably welded tobrackets 310 and 312, as shown. A low-friction sleeve 316 is providedfor sleeve 314 and is inserted therein.

[0037] Frame 142 has longitudinal side members 318 and 320 andcross-brace members 322 and 324. Cross-brace members 322 and 324 arepreferably welded to longitudinal side members 318 and 320, as shown. Apair of frame brackets 326 and 328 are preferably welded to longitudinalside member 318. The frame brackets 326 and 328 are spaced apart suchthat sleeve 314 can be inserted there between and further include guideholes or apertures such that a pin or bolt 330 can be inserted throughbracket 326, sleeve 314, and bracket 328. In this manner, pivot arm 132and its attachments can pivot around bolt 330 and are pivotally mountedto frame 142. Pivot arm 134 is similarly constructed and mounted toframe 142.

[0038] Referring now to FIGS. 4A through 4C, free body diagramsillustrating various centers of gravity and the forces acting onwheelchair 100 will now be described. In particular, FIG. 4A is a freebody diagram illustrating the forces acting on wheelchair 100 when thewheelchair is in static equilibrium. The various forces shown includeF_(p), F_(b), F_(s), F_(fc), F_(rc), and F_(w). More specifically, F_(p)is the force representing gravity acting on the center of gravity of aperson C_(gp) sitting in wheelchair 100. Similarly, F_(b) is the forcerepresenting gravity acting on the center of gravity of the batteriesC_(gb) used to power wheelchair 100. Resilient member or spring 144introduces a resilient force F_(s) acting on pivot arm 132 through itsconnection to the housing of drive motor 136. A second resilient memberor spring 146 (see FIG. 3) provides a similar force on pivot arm 134.Rear caster 110 has a force F_(rc) acting on its point of contact withthe ground. Front caster 106 has a force F_(fc) acting on its point ofcontact with the ground. Front caster 108 (not shown in FIG. 4A) has asimilar force acting on it as well. Drive wheel 102 has force F_(w)acting on its point of contact with the ground and drive wheel 104 alsohas a similar force acting thereon.

[0039] In wheelchair 100, the center of gravity of a person C_(gp)sitting in the wheelchair is preferably located behind a verticalcenterline 402 through pivotal connection P. Similarly, the center ofgravity of the batteries C_(gb) is located behind the verticalcenterline 402. As already described, it is possible to obtain betweenapproximately 80% to 95% weight distribution on drive wheels 102 and104, with the remainder of the weight being distributed between thefront casters 106 and 108 and the rear caster 110. As will be explainedin more detail, such an arrangement facilitates the raising and loweringof the front casters 106 and 108 during acceleration and deceleration ofthe wheelchair 100.

[0040] Under static equilibrium such as, for example, when the chair isat rest or not accelerating or decelerating as shown in FIG. 4A, the netrotational moment around pivotal connection P and pivot arms 132 and 134is zero (0) (i.e., ΣF_(n)r_(n)=0, where F is a force acting at adistance r from the pivotal connection P and n is the number of forcesacting on the wheelchair). Hence, pivot arms 132 and 134 do not tend torotate or pivot.

[0041] In FIG. 4B, wheelchair 100 is shown accelerating. The forces arethe same as those of FIG. 4A, except that an acceleration force F_(a) isacting on drive wheel 102. A similar force acts on drive wheel 104. Whenthe moment generated by the acceleration force F_(a) exceeds the momentgenerated by spring force F_(s), pivot arm 132 will begin to rotate orpivot such that front caster 106 begins to rise. As the moment generatedby the acceleration force F_(a) continues to increase over the momentgenerated by spring force F_(s), the pivot arm 132 increasingly rotatesor pivots thereby increasingly raising front caster 106 until themaximum rotation or pivot has been achieved. The maximum rotation orpivot is achieved when pivot arm 132 makes direct contact with frame 142or indirect contact such as through, for example, a pivot stop attachedto frame 142. Pivot arm 134 and front caster 108 behave in a similarfashion.

[0042] Hence, as the wheelchair 100 accelerates forward and the momentcreated by accelerating force F_(a) increases over the moment created byspring force F_(s), pivot arms 132 and 134 begin to rotate or pivotthereby raising front casters 106 and 108 off the ground. As described,it is preferable that front casters 106 and 108 rise between 1 and 6inches and most preferably between 1 and 4 inches off the ground so asto be able to traverse a curb or other obstacle of the same or similarheight.

[0043] Referring now to FIG. 4C, a free body diagram illustrating theforces acting on wheelchair 100 when the wheelchair is decelerating isshown. The forces are the same as those of FIG. 4A, except that adeceleration force F_(d) is acting on drive wheel 102 instead of anacceleration force F_(a). A similar force acts on drive wheel 104. Themoment generated by the deceleration force F_(d) causes pivot arm 132 torotate in the same direction as the moment generated by spring forceF_(s), i.e., clockwise as shown. If front caster 106 is not contactingthe ground, this pivot arm rotation causes front caster 106 to loweruntil it makes contact with the ground. If front caster 106 is alreadycontacting the ground, then no further movement of front caster 106 ispossible. Hence, when wheelchair 100 decelerates, front caster 106 isurged towards the ground. Pivot arm 134 and front caster 108 behave in asimilar manner.

[0044] The spring force F_(s) can be used to control the amount ofacceleration and deceleration that is required before pivot arm 132pivots and raises or lowers front caster 106. For example, a strong orweak spring force would require a stronger or weaker acceleration anddeceleration before pivot arm 132 pivots and raises or lowers frontcaster 106, respectively. The exact value of the spring force F_(s)depends on designer preferences and overall wheelchair performancerequirements for acceleration and deceleration. For example, the springforce F_(s) must be strong enough to keep chair 120 and the passengerfrom tipping forward due to inertia when the wheelchair is decelerating.It should also be noted that, in conjunction with the spring forceF_(s), the center of gravity of the person C_(gp) sitting in thewheelchair can be modified. For example, the center of gravity C_(gp)may be moved further rearward from vertical centerline 402 by movingchair 120 rearward along frame 142 with or without adjusting themagnitude of the spring force F_(s). Moreover, the position of pivotalconnection P may be moved along the length of pivot arms 132 and 134thereby changing the ratio of distances between the pivotal connection Pand the motor drive assemblies and casters 106 and 108 thereby resultingchanging the dynamics of the pivot arms and wheelchair. Hence, acombination of features can be varied to control the pivoting of pivotarms 132 and 132 and the raising and lowering of front casters 106 and108.

[0045] Referring now to FIGS. 5A through 5E, the curb-climbingcapability of wheelchair 100 will now be described. In FIG. 5A, thewheelchair 100 approaches a curb 502 of approximately 2 to 4 inches inheight. The wheelchair 100 is positioned so that front casters 106 and108 are approximately 6 inches from the curb 502. Alternatively,wheelchair 100 can be driven directly to curb 502 such that frontcasters 106 and 108 bump against curb 502 and are driven thereunto,provided the height of curb 502 is less than the axle height of frontcasters 106 and 108 (not shown).

[0046] Nevertheless, in FIG. 5B from preferably a standstill position,drive motors 136 and 138 are “torqued” so as to cause pivot arms 132 and134 to pivot about, for example, pin or bolt 330 and raise front casters106 and 108 off the ground. The torquing of drive motors 136 and 138refers to the process by which drive motors 136 and 138 are directed toinstantaneously produce a large amount of torque so that theacceleration force F_(a) creates a moment greater than the momentgenerated by spring force F_(s). Such a process is accomplished by thewheelchair's passenger directing the wheelchair to accelerate rapidlyfrom the standstill position. For example, a passenger can push hard andfast on the wheelchair's directional accelerator controller (not shown)thereby directing the wheelchair to accelerate forward as fast aspossible. As shown in FIG. 5B and as described in connection with FIGS.4A-4C, such “torquing” causes pivot arms 132 and 134 to pivot about pin330 thereby causing front casters 106 and 108 to rise. During torquing,the wheelchair 100 accelerates forward toward the curb 502 with thefront casters 106 and 108 in the raised position.

[0047] In FIG. 5C, front casters 106 and 108 have passed over curb 502.As front casters 106 and 108 pass over or ride on top of curb 502, drivewheels 102 and 104 come into physical contact with the rising edge ofcurb 502. Due to the drive wheels' relatively large size compared to theheight of curb 502, the drive wheels 102 and 104 are capable of engagingcurb 502 and driving there over—thereby raising the wheelchair 100 overcurb 502 and onto a new elevation. Once raised, the front casters 106and 108 are lowered as the inertial forces of the passenger and batteryapproach zero. These inertial forces approach zero when wheelchair 100either decelerates such as, for example, by engaging curb 502 or byaccelerating wheelchair 100 to its maximum speed (under a given loading)at which point the acceleration approaches zero and wheelchair 100approaches the state of dynamic equilibrium. Either scenario causespivot arms 132 and 134 to lower front casters 106 and 108 onto the newelevation.

[0048]FIG. 5D shows wheelchair 100 after the drive wheels 102 and 104have driven over curb 502 and onto the new elevation with front casters106 and 108 lowered. Rear caster 110 still contacts the previous lowerelevation. By such contact, rear caster 110 provides rearward stabilitypreventing wheelchair 100 from tipping backwards as the wheelchairclimbs the curb 502. FIG. 5E illustrates wheelchair 100 after rearcaster 110 has engaged and surmounted curb 502.

[0049] Hence, the present invention provides a feature by which thefront casters of a wheelchair can be raised and lowered when thewheelchair must climb or surmount a curb or obstacle. By raising thefront casters to an appropriate position, whether completely clear ofthe curb or obstacle height or partially clear thereof, the wheelchair'sdrive wheels can, in effect, drive the wheelchair over the curb orobstacle.

[0050] Referring now to FIGS. 6A through 6D, the curb-descendingcapability of wheelchair 100 will now be described. Referring nowparticularly to FIG. 6A, wheelchair 100 slowly approaches a curb 602,which represents a drop in elevation. In FIG. 6B, front casters 106 and108 have gone over curb 602 and are in contact with the new lowerelevation. As front casters 106 and 108 go over the curb or obstacle602, they are urged downward toward the new lower elevation by the forcegenerated by springs 144 and 146. This results in very little impact orfeeling of loss of stability to the wheelchair passenger because thewheelchair 100 stays substantially level as the front casters 106 and108 drop over curb 602 to the new lower elevation.

[0051] In FIG. 6C, drive wheels 102 and 104 have gone over curb 602 andare in contact with the new lower elevation. As drive wheels 102 and 104go over curb 602, wheelchair 100 is prevented from tipping forward bysprings 144 and 146 and front casters 106 and 108. More specifically,springs 144 and 146 urge the back of seat 120 rearward to counter anyforward tipping tendency that the wheelchair may exhibit. In addition orin the alternative, an electromechanical stop or spring dampener can beenergized by sensing inertial forces, angle of the wheelchair frame, orcurrent to or from the drive motors, which would prevent the wheelchairfrom tipping forward (not shown).

[0052] In FIG. 6D, rear caster 110 has gone over curb 602 and contactsthe new lower elevation. As rear caster 110 drops down over curb orobstacle 602, very little impact or instability is experienced by thewheelchair passenger because most of the wheelchair's weight (includingpassenger weight) is supported by drive wheels 102 and 104, which arealready on the new lower elevation. Hence, as rear caster 110 goes overcurb 602 and contacts the new lower elevation, the wheelchair passengerexperiences a low-impact transition between elevations.

[0053] Therefore, wheelchair 100 provides a stable, low-impact structureand method for climbing or descending over curb-like obstacles. Inclimbing curb-like obstacles, wheelchair 100 raises the front casters toa height sufficient for the front casters to go over the curb-likeobstacle and allow the wheelchair's drive wheels to engage the obstacle.The rear caster provides rearward stability during such curb-climbing.In descending curb-like obstacles, wheelchair 100 lowers the frontcasters over the obstacle to provide forward stability as the drivewheels drive over the obstacle. The resilient members or springs providerearward stability by urging the rear of the wheelchair's seat downwardto counter any forward tipping tendency that the wheelchair may exhibitwhen descending a curb or obstacle. Additionally, chair or seat 120 canbe moved rearward or tilted backward to increase wheelchair stabilitywhen descending a curb or obstacle.

[0054] Referring now to FIGS. 7 and 8, a second embodiment of acurb-climbing wheelchair 700 of the present invention is shown. Thewheelchair 700 has a pair of drive wheels 702 and 704, front casters 706and 708, rear caster 710, and front riggings 712 and 714. As inwheelchair 100, the front riggings 712 and 714 include footrests 716 and718 for supporting the feet of a passenger. The front riggings 712 and714 are preferably mounted so as to be able to swing away from the showncenter position to the sides of the wheelchair. Additionally, footrests716 and 718 can swing from the shown horizontal-down position to avertical-up position thereby providing relatively unobstructed access tothe front of the wheelchair.

[0055] The wheelchair 700 further includes a chair 720 having a seatportion 722 and a back portion 724 for comfortably seating a passenger.Chair 720 is adjustably mounted to frame 742 (see FIG. 8) so as to beable to move forward and backward on frame 742, thereby adjusting thepassenger's weight distribution and center of gravity relative to thewheelchair. As in wheelchair 100, chair 720 is preferably positionedsuch that a substantial portion of the wheelchair's weight when loadedwith a passenger is evenly distributed between drive wheels 702 and 704.For example, the preferred weight distribution of wheelchair 700 whenloaded with a passenger should be between 80% to 95% (or higher) ondrive wheels 702 and 704. The remainder of the weight being distributedbetween the rear and front casters. Armrests 726 and 728 are alsoprovided for resting the arms of a passenger or assisting a passenger inseating and unseating from chair 720.

[0056] The wheelchair 700 is preferably powered by one or more batteries730, which reside beneath the chair 720 and in-between drive wheels 702and 704. A pair of drive motors 736 and 738 (see FIG. 8) are used topower drive wheels 702 and 704. Drive motors 736 and 738 are preferablybrushless, gearless, direct-drive motors with their rotors eitherinternal or external to their stators. Drive motors 736 and 738 alsoeach include a fail-safe braking mechanism that includes a manualrelease mechanism (not shown). A control system and controller (notshown) interface batteries 730 to drive motors 736 and 738 so as toallow a passenger to control the operation of the wheelchair 700. Suchoperation includes directing the wheelchair's acceleration,deceleration, velocity, braking, direction of travel, etc.

[0057] Front casters 706 and 708 are attached to pivot arms 732 and 734,respectively. Rear caster 710 is attached to rear caster arms 740A and740B (see FIG. 8). While only one rear caster is shown, it should beunderstood that in the alternative two or more rear casters can also beprovided. As will be described in more detail, pivot arms 732 and 734are pivotally coupled to frame 742 for curb-climbing and descending,while rear caster arm 740A and 740B are rigidly coupled to frame 742.

[0058] The suspension and drive components of wheelchair 700 are furtherillustrated in the exploded prospective view of FIG. 9A. Morespecifically, pivot arm 732 has a base portion 906, an angled portion902 extending therefrom, and a motor mount bracket 910. The distal endof angled portion 902 includes a front swivel assembly 904 thatinterfaces with front caster 706. Base portion 706 has a portionincluding a hole 905 for pivot pin 922 and associated sleeve fittings.

[0059] The suspension further includes a coupling plate 914 forinterfacing front resilient assembly 931 to pivot arm 732. Couplingplate 914 is preferably rigidly affixed to pivot arm 732 via rigidtubular connection 916. Coupling plate 914 has a mounting bracket 918configured to receive a pivot pin for interfacing to front resilientassembly 931. Configured as such, pivot arm 732 and coupling plate movein unison about pivot pin or bolt 922 subject to the forces and momentsgenerated by front resilient assembly 931 and motor 736. Additionally,the suspension can further include a torsion member (not shown) betweenpivot arms 732 and 734 similar to the arrangement shown in FIG. 2B.

[0060] A resilient suspension member such as spring 920 extends betweenand is connected at its opposite ends to pivot arm 732 to a motor mount908. Motor mount 908 has a pivot connection 912 that pivotally couplesmotor mount bracket 910 to pivot arm 732 and coupling plate 914 via apivot pin. More specifically, motor mount 908 is pivotally received in aspace between motor mount bracket 910 and coupling plate 914. Motormount 908 further includes holes for fastening motor 136 thereto.Configured as such, motor 736 is pivotally coupled to pivot arm 732,which is itself pivotally coupled to frame 742.

[0061] Referring now to FIGS. 9A and 10A, front resilient assembly 931has a spring 938 that is indirectly coupled to frame 742 and couplingplate 914 via arcuate pivot brackets 932 and 934 and horizontal pivotbracket 936. Arcuate pivot brackets 932 and 934 are generally curved andhave holes in their distal portions. The holes are used for securingarcuate pivot brackets 932 and 934 to frame mounting bracket 940 and tohorizontal pivot bracket 936 via screws or pins. Spring 938 is coupledto the lower portions of arcuate pivot brackets 932 and 934 proximate toframe mounting bracket 940 and to one of a plurality of points shownbetween the distal portions of horizontal pivot bracket 936.

[0062] In this regard, horizontal pivot bracket 936 has a first distalportion having a pivot hole for interfacing with coupling plate 914 and,more particular, spring mounting bracket 918. The other distal portionof horizontal pivot bracket 936 has a plurality of mounting holes thatallow for the mounting of arcuate pivot brackets 932 and 934 in variouspositions. So configured front resilient assembly 931 is similar infunction to springs 144 and 146 of wheelchair 100. However, theconfiguration of linkages 932, 934, and 936 and spring 938 of frontresilient assembly 931 provide for a constant spring force over therange of pivoting of pivot arm 732.

[0063]FIGS. 11A through 11E and 12A through 12D illustrate the responseof the front resilient assembly 931 linkages with respect to wheelchair700 climbing and descending a curb-like obstacle.

[0064] Still referring to FIG. 9A, frame 742 includes longitudinal sidemembers 924 and 926 and cross-brace members 928 and 930. Pivot arm 732is pivotally mounted to side members 926 through pivot arm base member906 and pin 922. Motor 736 is pivotally mounted to pivot arm 732 throughmotor mount 908 and its pivot assembly 912. Since motor 736 is pivotalwith respect to pivot arm 732, spring 920 provides a degree ofsuspension between the two pivotal components. Additionally, since pivotarm 732 pivots with respect to frame 742, spring 938 and associatedvertical and horizontal pivot brackets 934, 936, and 938, respectively,urge pivot arm 732 such that front caster 706 is urged downward towardthe riding surface. This is similar in functionality to spring 144 ofwheelchair 100.

[0065]FIG. 9B is an enlarged view of portion 942 of FIG. 9A. Morespecifically, portion 942 shows pivot arm 734 and its associatedcomponents, which are similarly configured to pivot arm 732 and itsassociated assemblies, in their assembled positions on frame 742.

[0066] Referring now to FIGS. 10A through 10C, free body diagramsillustrating various centers of gravity and the forces acting onwheelchair 700 will now be described. In particular, FIG. 10A is a freebody diagram illustrating the forces acting on wheelchair 700 when thewheelchair is in static equilibrium. The various forces shown includeF_(p), F_(b), F_(s), F_(fc), F_(rc), and F_(w). As described in FIGS.4A-4C, F_(p) is the force representing gravity acting on the center ofgravity of a person C_(gp) sitting in wheelchair 700. Similarly, F_(b)is the force representing gravity acting on the center of gravity of thebatteries C_(gb) used to power wheelchair 100. Spring 944 introduces aforce F_(s) acting on pivot arm 732. Spring 938 (see FIG. 9A) provides asimilar force on pivot arm 732. Rear caster 710 has a force F_(rc)acting on its point of contact with the ground. Front caster 708 has aforce F_(fc) acting on its point of contact with the ground. Frontcaster 706 (see FIG. 9A) has a similar force acting on it as well. Drivewheel 704 has force F_(w) acting on its point of contact with the groundand drive wheel 702 (see FIG. 9A) also has a similar force actingthereon.

[0067] In wheelchair 700, the center of gravity C_(gp) of a personsitting in the chair is preferably located just behind a verticalcenterline 1002 through pivotal connection P. Similarly, the center ofgravity C_(gb) of the batteries is located behind the verticalcenterline 1002. As already described, it is possible to obtain betweenapproximately 80% to 95% weight distribution on drive wheels 702 and704, with the remainder of the weight being distributed between thefront casters 706 and 708 and the rear caster 710. As will be explainedin more detail, such an arrangement facilitates the raising and loweringof the front casters 706 and 708 during acceleration and deceleration ofthe wheelchair 700.

[0068] Under static equilibrium such as, for example, when the chair isat rest or not accelerating or decelerating as shown in FIG. 10A, thenet rotational moment around pivotal connection P and pivot arms 732 and734 is zero (0) (i.e., ΣF_(n)r_(n)=0, where F is a force acting at adistance r from the pivotal connection P and n is the number of forcesacting on the wheelchair). Hence, pivot arms 732 and 734 do not tend torotate or pivot.

[0069] In FIG. 10B, wheelchair 700 is shown accelerating. The forces arethe same as those of FIG. 10A, except that an acceleration force F_(a)is acting on drive wheel 704. A similar force acts on drive wheel 702.When the moment generated by the acceleration force F_(a) exceeds themoment generated by spring force F_(s), pivot arm 734 will begin torotate or pivot such that front caster 708 begins to rise. As the momentgenerated by the acceleration force F_(a) continues to increase over themoment generated by spring force F_(s), pivot arm 734 increasinglyrotates or pivots thereby increasingly raising front caster 708 untilthe maximum rotation or pivot has been achieved. The maximum rotation orpivot is achieved when pivot arm 734 makes direct contact with frame 742or indirect contact such as through, for example, a pivot stop attachedto frame 742. Pivot arm 734 and front caster 708 behave in a similarfashion.

[0070] Hence, as the wheelchair 700 accelerates forward and the( momentcreated by accelerating force F_(a) increases over the moment created byspring force F_(s), pivot arms 732 and 734 being to rotate or pivotthereby raising front casters 706 and 708 off the ground. As described,it is preferable that front casters 706 and 708 rise between 1 and 6inches off the ground so as to be able to overcome a curb or otherobstacle of the same or similar height.

[0071] Referring now to FIG. 10C, a free body diagram illustrating theforces acting on wheelchair 700 when the wheelchair is decelerating isshown. The forces are the same as those of FIG. 10A, except that adeceleration force F_(d) is acting on drive wheel 702 instead of anaccelerating force F_(a). A similar force acts on drive wheel 702. Themoment generated by the deceleration force F_(d) causes pivot arm 734 torotate in the same direction as the moment generated by spring forceF_(s), i.e., clockwise as shown. If front caster 708 is not contactingthe ground, this pivot arm rotation causes front caster 708 to loweruntil it makes contact with the ground. If front caster 708 is alreadycontacting the ground, then no further movement of front caster 708 ispossible. Hence, when wheelchair 700 decelerates, front caster 708 isurged clockwise or towards the ground. Pivot arm 732 and front caster706 behave in a similar manner.

[0072] As with wheelchair 100, the spring force F_(s) can be used tocontrol the amount of acceleration and deceleration that is requiredbefore pivot arm 734 pivots and raises or lowers front caster 708. Forexample, a strong or weak spring force would require a stronger orweaker acceleration and deceleration before pivot arm 734 pivots andraises or lowers front caster 708, respectively. The exact value of thespring force F_(s) depends on designer preferences and overallwheelchair performance requirements for acceleration and deceleration.For example, the spring force F_(s) must be strong enough to keep chair720 and the passenger from tipping forward due to inertia when thewheelchair is decelerating. Additionally, because horizontal pivotbracket 936 has a plurality of mounting holes (see FIG. 9A, for example)for mounting vertical pivot brackets 932 and 934, the amount of springforce F_(s) applied to the pivot arms can also be controlled by theappropriate choice of mounting for such brackets. It should also benoted that, either alone or in conjunction with the spring force F_(s)and the vertical and horizontal pivot bracket configuration, the centerof gravity of the person C_(gp) sitting in the wheelchair can bemodified. For example, the center of gravity C_(gp) may be moved furtherrearward from vertical centerline 1002 with or without adjusting themagnitude of the spring force F_(s). Hence, a combination of featurescan be varied to control the pivoting of pivot arms 732 and 732 and theraising and lowering of front casters 706 and 708.

[0073] Referring now to FIGS. 11A through 11E, the curb-climbingcapability of wheelchair 700 will now be described. In FIG. 11A, thewheelchair 700 approaches a curb 1102 of approximately 3 to 6 inches inheight. The wheelchair 700 is positioned so that front casters 706 and708 are approximately 6 inches from the curb 1102. Alternatively,wheelchair 700 can be driven directly to curb 1102 such that frontcasters 706 and 708 bump against curb 1102 and are driven thereunto,provided the height of curb 1102 is less than the axle height of frontcasters 706 and 708 (not shown).

[0074] Nevertheless, in FIG. 11B from preferably a standstill position,drive motors 736 and 738 are “torqued” so as to cause pivot arms 732 and734 to pivot about, for example, pin or bolt 922 and raise front casters706 and 708 off the ground. As described earlier, the torquing of drivemotors 736 and 738 refers to the process by which drive motors 736 and738 are directed to instantaneously produce a large amount of torque sothat the acceleration force F_(a) creates a moment greater than themoment generated by spring force F_(s). Such a process is accomplishedby the wheelchair's passenger directing the wheelchair to acceleraterapidly from the standstill position. For example, a passenger can pushhard and fast on the wheelchair's directional accelerator controller(not shown) thereby directing the wheelchair to accelerate forward asfast as possible. As shown in FIG. 11B and as described in connectionwith FIGS. 10A-10C, such “torquing” causes pivot arms 732 and 734 topivot about pin 922 thereby causing front casters 706 and 708 to rise.During torquing, the wheelchair 700 accelerates forward toward the curb1102 with the front casters 706 and 708 in the raised position.

[0075] In FIG. 1C, front casters 706 and 708 have passed over curb 1102.As front casters 706 and 708 pass over or ride on top of curb 1102,drive wheels 702 and 704 come into physical contact with the rising edgeof curb 1102. Due to the drive wheels' relatively large size compared tothe height of curb 1102, the drive wheels 702 and 704 are capable ofengaging curb 1102 and driving there over—thereby raising the wheelchair700 over curb 1102 and onto a new elevation. As drive wheels 702 and 704engage curb 1102, suspension spring 920 cushions the impact of thetransition. Once raised, the front casters 706 and 708 are lowered asthe inertial forces of the passenger and battery approach zero. Theseinertial forces approach zero when wheelchair 700 either deceleratessuch as, for example, by engaging curb 1102 or by acceleratingwheelchair 700 to its maximum speed (under a given loading) at whichpoint the acceleration approaches zero and wheelchair 700 approaches thestate of dynamic equilibrium. Either scenario causes pivot arms 732 and734 to lower front casters 706 and 708 onto the new elevation.

[0076]FIG. 11D shows wheelchair 700 after the drive wheels 702 and 704have driven over curb 1102 and onto the new elevation with front casters706 and 708 lowered. Rear caster 710 still contacts the previous lowerelevation. By such contact, rear caster 710 provides rearward stabilitypreventing wheelchair 700 from tipping backwards as the wheelchairclimbs the curb or obstacle 1102. FIG. 11E illustrates wheelchair 700after rear caster 710 has engaged and surmounted curb or obstacle 1102.FIGS. 12A, 12B, 12C, 12D, and 12E correspond to enlarge portions ofFIGS. 11A, 11B, 11C, 11D, and 11E, respectively, particularly showingthe orientation and range of motion experienced by front resilientassembly 931 as the wheelchair climbs a curb.

[0077] Hence, the embodiment of wheelchair 700 provides a feature bywhich the front casters of a wheelchair can be raised and lowered whenthe wheelchair must climb or surmount a curb or obstacle. By raising thefront casters to an appropriate position, whether completely clear ofthe curb or obstacle height or partially clear thereof, the wheelchair'sdrive wheels can, in effect, drive the wheelchair over the curb orobstacle.

[0078] Referring now to FIGS. 13A through 13D, the curb descendingcapability of wheelchair 700 will now be described. Referring nowparticularly to FIG. 13A, wheelchair 700 slowly approaches a curb 1302,which represents a drop in elevation. In FIG. 13B, front casters 706 and708 have gone over curb 1302 and are in contact with the new lowerelevation. As front casters 706 and 708 go over curb 1302, they areurged downward toward the new lower elevation by the force generated bysprings 938 and 944. This results in very little impact or feeling ofloss of stability to the wheelchair passenger because the wheelchair 700stays substantially level as the front casters 706 and 708 drop overcurb 1302 to the new lower elevation.

[0079] In FIG. 13C, drive wheels 702 and 704 have gone over curb 1302and are in contact with the new lower elevation. As drive wheels 702 and704 go over curb or obstacle 1302, suspension springs such as spring 920cushion the impact of such a transition. Also as drive wheels 702 and704 go over curb 1302, wheelchair 700 is prevented from tipping forwardby springs 938 and 944 and front casters 706 and 708. More specifically,springs 938 and 944 urge the front of frame 742, through frame mountingbracket 940 (see FIGS. 9 and 10), upward to counter any forward tippingtendency that the wheelchair may exhibit.

[0080] In FIG. 13D, rear caster 710 has gone over curb 1302 and contactsthe new lower elevation. As rear caster 710 drops down over curb 1302,very little impact or instability is experienced by the wheelchairpassenger because most of the wheelchair's weight (including passengerweight) is supported by drive wheels 702 and 704, which are already onthe new lower elevation. Hence, as rear caster 710 goes over curb 1302and contacts the new lower elevation, the wheelchair passengerexperiences a low-impact transition between elevations.

[0081] Therefore, wheelchair 700 provides a stable, low-impact structureand method for climbing or descending over curb-like obstacles. Inclimbing curb-like obstacles, wheelchair 700 raises the front casters toa height sufficient for the front casters to go over the curb-likeobstacle and allow the wheelchair's drive wheels to engage the obstacle.The rear caster provides rearward stability during such curb-climbing.In descending curb-like obstacles, wheelchair 700 lowers the frontcasters over the obstacle to provide forward stability as the drivewheels drive over the obstacle. Suspension springs associated with thedrive wheels provide for low-impact transitions for the passengerbetween elevations representing curbs or obstacles. Springs associatedwith the front casters provide forward stability by urging the front ofthe wheelchair's frame upward to counter any forward tipping tendencythat the wheelchair may exhibit when descending a curb or obstacle.

[0082] While the present invention has been illustrated by thedescription of embodiments thereof, and while the embodiments have beendescribed in considerable detail, it is not the intention of theapplicants to restrict or in any way limit the scope of the appendedclaims to such detail. Additional advantages and modifications willreadily appear to those skilled in the art. For example, the pivot armscan be made from a plurality of components having differing geometry,the wheelchair may or may not include spring forces acting on the pivotarms, the invention can be applied to rear-wheel and front-wheel drivewheelchairs, elastomeric resilient members can be used instead of or incombination with springs, electrically adjustable spring tension devicescan be included with the springs, etc. Therefore, the invention, in itsbroader aspects, is not limited to the specific details, therepresentative apparatus, and illustrative examples shown and described.Accordingly, departures may be made from such details without departingfrom the spirit or scope of the applicant's general inventive concept.

We claim:
 1. A suspension system for a vehicle comprising: a frame; atleast one rear caster coupled to the frame; first and second pivotingassemblies, each pivoting assembly comprising at least one pivot armhaving: a first portion pivotally coupled of the frame; at least onefront caster coupled to a second portion of the pivot arm and normallyin contact with a supporting surface of the vehicle; a drive wheelcoupled to a third portion of the pivot arm; wherein the first portionis coupled proximate to the frame member so as to cause 80-95% of theweight of the vehicle being supported by the drive wheels and theremaining % weight of the vehicle being supported at least by the atleast one front caster and urging the at least one front caster intocontact with the supporting surface; and wherein the pivot arm furthercomprises a pivoting behavior responsive to a force generated by thedrive wheel, the pivoting behavior raising the at least one front casterfrom the supporting surface of the vehicle and at least partiallyshifting the % weight being supported by the at least one front casterto the at least one rear caster so that the weight of the vehicle issupported by the drive wheels and the at least one rear caster.
 2. Thesuspension system of claim 1 wherein the frame member comprises an upperframe member having a bracket depending downwardly and away from theupper frame member, the bracket including a first distal portion coupledto the upper frame member and a second distal portion terminating inspace and free of physical coupling or attachment.
 3. The suspensionsystem of claim 2 wherein the pivot arm further comprises a base memberand wherein the first portion is disposed on the base member andpivotally couples the base member to the bracket so that the base memberis substantially disposed below the upper frame member.
 4. Thesuspension system of claim 2 wherein the pivot arm further comprises abase member and an extension member and wherein the first portion isdisposed on the base member and pivotally couples the base member to thebracket so that the base member is substantially disposed below theupper frame member and the extension member projects into space in adirection away from the upper frame member.
 5. The suspension system ofclaim 1 wherein the pivot arm comprises an attachment member for atorsion bar.
 6. The suspension system of claim 5 further comprising atorsion bar coupled to the attachment member of the pivot arm.
 7. Asuspension system for a vehicle comprising: a frame having an upperframe member having a bracket depending downwardly and away from theupper frame member, the bracket including a first distal portion coupledto the upper frame member and a second distal portion terminating inspace and free of physical coupling or attachment; at least one rearcaster coupled to the frame; first and second pivoting assemblies, eachpivoting assembly comprising at least one pivot arm having: a firstportion pivotally coupled of the frame; at least one front castercoupled to a second portion of the pivot arm and normally in contactwith a supporting surface of the vehicle; a drive wheel coupled to athird portion of the pivot arm; a base member; and wherein the firstportion is disposed on the base member and pivotally couples the basemember to the bracket so that the base member is substantially disposedbelow the upper frame member.
 8. The suspension system of claim 7wherein the pivot arm further comprises an extension member coupled tothe base member and wherein the extension member projects into space ina direction away from the upper frame member.
 9. The suspension systemof claim 7 wherein the extension member is coupled in angled relation tothe base member.
 10. The suspension system of claim 7 wherein the basemember comprises an upper section and a lower section and the firstportion pivotally coupling the pivot arm to the frame is disposed on theupper section.
 11. The suspension system of claim 7 wherein the firstportion is coupled proximate to the upper frame member so as to cause80-95% of the weight of the vehicle being supported by the drive wheelsand the remaining % weight of the vehicle being supported at least bythe at least one front caster and urging the at least one front casterinto contact with the supporting surface.
 12. The suspension system ofclaim 11 wherein the pivot arm further comprises a pivoting behaviorresponsive to a force generated by the drive wheel, the pivotingbehavior raising the at least one front caster from the supportingsurface of the vehicle and at least partially shifting the % weightbeing supported by the at least one front caster to the at least onerear caster so that the weight of the vehicle is supported by the drivewheels and the at least one rear caster.
 13. The suspension system ofclaim 7 wherein the pivot arm comprises an attachment member for atorsion bar.
 14. The suspension system of claim 13 further comprising atorsion bar coupled to the attachment member of the pivot arm.
 15. Asuspension system for a vehicle comprising: (a) a frame; (b) a pivotingassembly having at least one pivot arm, the at least one pivot armcomprising: (i) a first portion pivotally coupled to the frame; and (ii)a second portion having at least one front caster attached thereto forcontacting a supporting surface of the wheelchair; (c) a drive assemblycoupled to a third portion of the at least one pivot arm; (d) at leastone rear caster coupled to said frame; and wherein the first portion ispivotally coupled proximate to the frame so as to cause 80-95% of theweight of the vehicle being supported by the drive wheels and theremaining % weight of the vehicle being supported at least by the atleast one front caster thereby urging the at least one front caster intocontact with the supporting surface; and wherein said at least one pivotarm pivots in response to a force generated by the drive assemblythereby raising the at least one front caster from the supportingsurface to assist the vehicle in traversing obstacles.
 16. Thesuspension system of claim 15 wherein the frame member comprises anupper frame member having a bracket depending downwardly and away fromthe upper frame member, the bracket including a first distal portioncoupled to the upper frame member and a second distal portionterminating in space and free of physical coupling or attachment. 17.The suspension system of claim 16 wherein the pivot arm furthercomprises a base member and wherein the first portion is disposed on thebase member and pivotally couples the base member to the bracket so thatthe base member is substantially disposed below the upper frame member.18. The suspension system of claim 16 wherein the pivot arm furthercomprises a base member and an extension member and wherein the firstportion is disposed on the base member and pivotally couples the basemember to the bracket so that the base member is substantially disposedbelow the upper frame member and the extension member projects intospace in a direction away from the upper frame member.
 19. Thesuspension system of claim 15 wherein the pivot arm comprises anattachment member for a torsion bar.
 20. The suspension system of claim19 further comprising a torsion bar coupled to the attachment member ofthe pivot arm.